The present invention relates to a charge phase control device for an ink jet printing or recording device of the type wherein the ink droplet charging phase is controlled.
In the ink jet printing or recording device, ink under pressure is fed to an ink head and the exciting signal of a predetermined frequency is impressed on an oscillator of the ink head so that the ink emerging from the tip of a nozzle may be formed into a train of ink droplets. The ink droplets are selectively charged positively or negatively in response to the video signal in the recording mode when they pass through the charging electrode. The positively or negatively charged ink droplets are deflected by a pair of deflection electrodes and attracted toward a recording medium so as to be recorded thereon as dots, but the uncharged ink droplets are trapped in a gutter.
In the ink jet printing or recording device of the type described, the formation of an ink droplet at the tip of the nozzle and the charging by the charging electrode must be precisely synchronized with each other so that the ink droplets may be correctly positively or negatively charged in response to the video signal which represents the position of a pattern element. In order to attain this synchronization, there has been devised and demonstrated a charge phase control device for an ink jet printing or recording device as disclosed in for instance Japanese Patent Publication Nos. 1972-43448 and 1972-43450. In this charge control device a train of ink droplets emitted from the tip of a nozzle is intermittently charged negatively or positively in the phase detection mode prior to the recording or printing. The negatively or positively charged ink droplets and the uncharged attract each other and coalesce into large ink droplets so that the number of ink droplets per unit of time is reduced. There may be established a one-to-one correspondence between this reduction in number of ink droplets and the frequency of intermittent energizations of the charging electrode. Therefore when the number of ink droplets counted by an ink droplet detecting and counting means has a predetermined relationship with the frequency of the intermittent energizations of the charging electrode, the ink droplet formation and the ink drop charging are detected as synchronized. On the other hand when the ink droplets do not coalesce so that the number of the ink droplets per unit of time will not be reduced, it is detected that the ink droplet formation and the ink droplet charging are not synchronized with each other. Thus the charging phase is controlled accordingly.
However when the ink droplets are alternately charged negatively or positively as described above, the force acting between the adjacent charged and uncharged ink droplets is weak (because each of the uncharged ink droplets is charged positively or negatively to more than 10% of the charge on the preceding ink droplet which is negatively or positively charged). As a result the ink droplets must travel over a relatively long distance before they can coalesce, and consequently the distance between the ink nozzle and the ink droplet detecting means must be increased accordingly with the resultant increase in size and weight of the ink jet printing or recording device. The charge phase control device of the type described has a further defect that since the phase detection is made with the ink head at its initial position, the ink head must be returned to the initial position from a recording position when the phase detection is made during printing or recording. As a result, a recording or printing interruption becomes longer.
In order to overcome the above problems, there has been proposed a method (as is disclosed in IBM Technical Disclosure Bulletin Vol. 16 No. 12, May 1974, PP. 3877-3878) wherein in the phase detection mode the charged ink droplets are deflected to pass through a photosensor disposed immediately before a gutter so that whether the phase synchronization is correct or not is detected in response to the output from the photosensor. Since the photosensor must be located in the path of the ink droplets where they are caused to be deflected and because the ink droplets are small, the slit of the photosensor must be made narrow. This method has a further defect that a phase detection voltage must be adjusted and set to a predetermined level with an extremely higher degree of accuracy.